I am the fortunate owner of both a Seca 700 Race and Seca 400 (ok so one of them is actually my wife's). I thought I would share my comparisons for all:
The beam pattern of these two lights is almost identical, although the 700 does not have as much of a hotspot in the centre, compared to the 400. The 700 is a bit brighter, and projects further, although the difference is not exactly overwhelming.
The major difference between these two lights is the colour. The 400 emits a bluish/white light, as I expected from an LED. However, the 700 emits a very yellow beam. Has anyone else encountered this? I'm almost wondering if there is something defective, because the difference between the two beams is quite striking. I have a friend with the 400, and his light is also bluish/white. I wish I could compare my 700 with someone else, because this yellow light is really strange to me.
Also, to dispense with the rumour once and for all: Using the 700 battery in the Seca 400 does not change it's light output... at least not visibly, although it would presumably burn for twice as long. Conversely, I guess you could use the 400 battery on the 700 to save weight if you were just going for a quick rip.
I did try using both together (one on the bar, one on the helmet). It was ridiculously bright and I had to be careful not to point them at my dog for fear of blinding him!
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Thread: Seca 400 vs 700 Race